1. Technical Field
The present invention relates to a resolver digital converter, and particularly to a resolver digital converter having a failure detection function.
2. Related Art
In a servo control system, a rotation angle sensor is required in order to detect a rotation angle and perform feedback control. Also, in control of a brushless motor, it is necessary to pass a current through a coil of the motor according to a rotation angle of the motor, so that the rotation angle sensor is required as well as the servo control system.
As this rotation angle sensor, in the related art, a resolver constructed of only a winding has been widely used because of environment resistance, toughness resulting from the simple configuration. This resolver is equivalent to a transformer in principle, and differs from the transformer structurally in that an iron core is divided into a rotor and a stator.
Also, in a servo control system applied to motor-driven power steering, x-by-wire, particularly, steer-by-wire, fly-by-wire, etc., a failure detection function is required in order to require safety and reliability.
Also, a resolver digital converter for making conversion into a rotation angle based on a signal from a resolver and doing input to a microcomputer etc. as digital data has been developed in recent years (for example, see Smartcoder, AU6802 Catalogue, Tamagawa Seiki Co., Ltd., T12-1591N2 (Apr. 20, 2002), which is hereinafter referred as Non-patent Reference 1). In the resolver digital converter according to this Non-patent Reference 1, a feedback loop for making a correction to an estimation value φ of a rotation angle by a residual ε of a result of performing calculation of an input signal and sin(φ), cos(φ) generated by the estimation value φ of the rotation angle is formed and the estimation value φ of the rotation angle is converged on an actual rotation angle θ. The present method further has a function of notifying the microcomputer that a failure occurs when the residual ε exceeds a certain value.
Also, a failure is detected by using characteristics of a trigonometric function of sin2θ+cos2θ=1 which is anoutput of a resolver (for example, see JP-A-9-280890 (Page 4, FIG. 1) which is hereinafter referred as Patent Reference 1).
According to Non-patent Reference 1, when a signal outputted from the resolver or a signal inputted to the resolver is blocked, the signal outputted from the resolver becomes abnormal and a relation between sin(θ) and cos(θ) used as a premise of convergence of a feedback loop does not hold. As a result of this, the residual ε does not converge and the residual ε becomes large and notification of a failure is provided from the resolver digital converter to the microcomputer. Also, when a feedback loop for obtaining an estimation value φ of a rotation angle including a resolver digital conversion part does not function correctly, the residual E does not converge similarly and the residual ε becomes large and error detection signal is provided from the resolver digital converter to the microcomputer.
However, according to Non-patent Reference 1, when a failure (open, short failure, permanent failure of output) of a function of notifying the microcomputer of occurrence of a failure and a function of determining that the residual ε exceeds a certain value occurs, there is a problem that the microcomputer cannot be notified of the occurrence of the failure.
Further, there is a failure mode in which a portion of a winding shorts out (a layer short) in the resolver, and according to Non-patent Reference 1, a failure is not detected in a state of specializing in the layer short and, so that there is a problem that there is a limit to accuracy of layer short failure detection.
Also, according to Patent Reference 1, when a signal outputted from the resolver or a signal inputted to the resolver is blocked, the signal outputted from the resolver becomes abnormal and a relation between sin(θ) and cos(θ) does not hold. As a result of this, a value of sin2θ+cos2θ deviates from 1, so that it is detected as a failure and a microcomputer can be notified. Also, in this Patent Reference 1, a failure is detected in a state of specializing in the relation between sin(θ) and cos(θ), so that unbalance of a level of sin(θ) and cos(θ) because of a layer short which is a failure mode in which a portion of a winding shorts out can be detected exactly. However, in the case of calculating sin2(θ)+cos2(θ), computation of square of a variable is required and this computation leads to a processing load of the microcomputer and particularly in the case of using a processor of the limit of capability from an economical standpoint, there is a problem that processing cannot be performed.
Further, according to Patent Reference 1, a detectable failure is only a failure about signal waveform abnormality in which a signal from or to the resolver is blocked, and there is a problem that a failure of the resolver digital conversion function itself which is a process of obtaining θ cannot be detected.
Also, a failure capable of providing notification to the microcomputer is:                (1) a failure about signal waveform abnormality in which a signal outputted from the resolver or a signal inputted to the resolver is blocked, etc.,        (2) a failure of a function of detecting the failure of (1) and notifying the microcomputer,        (3) a failure of the resolver digital conversion function itself which is a process of obtaining θ based on a signal outputted from the resolver, and,        (4) a failure of a function of detecting the failure of (3) and notifying the microcomputer.        
Even in the case of combining Non-patent Reference 1 with Patent Reference 1, these failures are not detected.